1. Field of the Invention
The present invention relates to an exposure method and an exposure device for transferring a mask pattern onto a photosensitive substrate during certain photolithographic processes. Such processes are typically used when manufacturing semiconductor devices, image pickup devices (CCDs, etc.), liquid-crystal display elements, thin-film magnetic heads, and so on. The use of such a method and such a process is particularly favorable when a scanning type exposure device, such as step-and-scan system, is used to perform exposure operations.
2. Description of Related Art
It is known, when manufacturing semiconductor devices, to use a reduced projection type exposure device, or stepper, in a step-and-repeat or batch exposure system for transferring a reticle pattern or mask to individual shot areas of a wafer or glass plate forming a photosensitive substrate. A projection exposure device for use in a step-and-scan system has also been proposed. In this type of system, a reduced image of the reticle pattern is successively transferred to each shot area of the wafer by scanning the reticle and wafer synchronously with respect to the projection optical system. In this way, the need for enlarging the area of the transfer target pattern is met.
In a scanning exposure type projection exposure device used in a step-and-scan system, it is necessary to move the wafer stage at a fixed scanning speed V.sub.w during exposure in order to provide a prescribed exposure value with respect to the photoresist on the wafer. It is also necessary to move the reticle stage in the corresponding direction by XW/.beta. synchronously with the movement of the wafer stage position XW, where the projection magnification from the reticle of the optical projection system to the wafer is B, in order to keep distortion and resolution of the reticle pattern image on the wafer within a prescribed margin. Therefore, the scanning speed of the reticle stage becomes V.sub.w /.beta. when the scanning speed of the wafer stage is V.sub.w.
FIG. 6 shows the change in speed V of the wafer stage with respect to time t when executing exposure in the one shot area with a projection exposure device for a step-and-scan system. In FIG. 6, the wafer stage is accelerated from rest to a scanning speed V.sub.w within an acceleration time TA. The positional misregistration of the reticle and wafer is held to a prescribed margin within the subsequent synchronous settling time TB. Exposure is executed by irradiating illuminating light during the next exposure time TC.
Assuming that the aforementioned acceleration time is designated TA, the exposure time is designated TC, the average time necessary for stepping between the shot areas and deceleration of the wafer stage is a shot processing time designated TS, and the loading time of the wafer is a wafer processing time designated TL, then the throughput, or exposure wafer count per unit time, N may be expressed as EQU N=C/{n.times.(TA+TB+TC+TS)+TL} (1)
In this equation, C is the unit time and n is the shot area count per wafer. It is apparent from equation (1) that the throughput N improves by reducing the exposure time TC during which illuminating light is actually irradiated on each shot area.
In the scanning exposure system mentioned above, the throughput N can be improved by reducing the actual exposure time TC in each shot area of the wafer. However, an inability to enhance throughput (N) to a desired level has existed since the scanning speed has had an upper limit determined by the wafer and the reticle stages.
When an exposure time with respect to any arbitrary point on the wafer is designated T.sub.w, the necessary exposure value of the photoresist on the wafer is designated E (mJ/cm.sup.2), the scanning speed of the wafer stage is designated V.sub.w (mm/sec), the maximum value of the scanning speed thereof is V.sub.Wmax, the slit width, in the scanning direction, of the slit shaped exposure field on the wafer is designated D, and the illumination intensity within the exposure field is designated P (mW/cm.sup.2), the following relationship must be established.
T.sub.w =E/P=D/V.sub.w.gtoreq.V.sub.Wmax (2)
If the slit width D is considered to be a fixed value in order to satisfy equation (2), then it is necessary to increase a scanning speed V.sub.w when the resist sensitivity is high and the necessary exposure value E is low. On the other hand, it is necessary to decrease the scanning speed V.sub.w when the resist sensitivity is low and the necessary exposure value E is high. The scanning speed V.sub.w, however, cannot exceed the maximum value V.sub.wmax obtained by the mechanism. When the resist sensitivity is a prescribed high sensitivity and the necessary exposure value E becomes a prescribed value E.sub.sa, the scanning speed V.sub.w reaches its maximum value V.sub.wmax. For example, when the maximum value of an illumination intensity P is P.sub.max, the prescribed value E.sub.sa of the necessary exposure value E can be described as EQU E.sub.sa =P.sub.max.cndot.D/V.sub.Wmax (3)
The scanning speed V.sub.w is fixed at a maximum value (V.sub.wmax) when the resist sensitivity is high and the necessary exposure value E is less than a prescribed value E.sub.sa. It is necessary, therefore, to reduce the illumination intensity P in order to satisfy equation (2) and obtain the necessary exposure value. When the scanning speed V.sub.w is fixed at the maximum value (V.sub.wmax), the exposure time T.sub.w to one arbitrary point on the wafer is fixed at D/V.sub.Wmax according to equation (2). The actual exposure time (TC) with respect to each shot area in equation (1) is fixed at a given minimum value, and the throughput (N) reaches an upper limit.
FIG. 7 shows the relationship of the throughput to the resist sensitivity. The horizontal axis in FIG. 7 is the necessary exposure value E (mJ/cm.sup.2) of the photoresist and the vertical axis is the throughput N (in arbitrary units) obtained from equation (1). FIG. 7 shows that the throughput N is high until the necessary exposure value E reaches the prescribed value E.sub.sa. Throughput N remains at a fixed value N1 during this time. The throughput N gradually decreases when the necessary exposure value E is above the prescribed value E.sub.sa. Therefore, the throughput, up to a given level, during the exposure process of the semiconductor element cannot be improved when using a photoresist of high sensitivity.
The sensitivity and the resolution oppose each other so that as the sensitivity becomes higher, the resolution becomes lower. In the layers of the semiconductor device, a photoresist of high resolution and low sensitivity is used in what is referred to as the "critical layer". The critical layer is a layer in the semiconductor device in which the superimposition precision of the pattern is rigorous and the minimum line width is, for example, 0.3 .mu.m. A photoresist of low resolution and high sensitivity is used in another layer, referred to as the "noncritical layer", in which the minimum line width is greater than 0.5 .mu.m and the superimposition precision of the pattern is relatively relaxed. When a photoresist of high sensitivity and large line width is used, an upper limit in the throughput is created.